Contaminants, contained in sewage and wastewater, are generally classified into organic matters and nitrogen/phosphorous as nutrient salts.
Particularly, when untreated nitrogen and phosphorous as nutrient salts are discharged to rivers, eutrophication may be caused, and red tides may occur in oceans. To address these issues, advanced biological treatment processes for treating nitrogen and phosphorous are introduced.
For example, a nitrogen removing process as one of advanced biological treatment processes is divided into nitrification for converting organic nitrogen or ammoniacal nitrogen in sewage/wastewater into nitrate nitrogen in an anoxic state, and denitrification for converting the nitrate nitrogen generated through the nitrification into nitrogen gas in an anaerobic state. Nitrogen is removed from the sewage/wastewater through the nitrification and the denitrification.
Examples of such a nitrogen removing process include a Wuhman process, a Ludzack-Ettinger process, a Bardenpho process, a packed bed denitrification reactor process, an intermittent aeration activated sludge process, a sequencing batch reactor (SBR) process, and an oxidation ditch process.
In a phosphorous removing process, organic phosphorous is accumulated in the form of phosphate within cells in an aerobic state, and is discharged in the form of phosphate from microorganisms in an anaerobic state.
The discharged phosphate is excessively absorbed and accumulated in microorganisms in an aerobic state, and then, the microorganisms are discharged as waste activated sludge, thereby removing phosphorous from sewage/wastewater. Examples of such a phosphorous removing process include A/O, A2/O, UCT, and VIP.
Unlike the variable level sequencing batch reactor (SBR) process in which treating target water is successively introduced and discharged, a constant level sequencing batch reactor (CSBR™) process maintained at a constant level has been recently used to remove contaminants from sewage/wastewater. This CSBR process is disclosed, for example, in U.S. Pat. No. 5,902,484. In the CSBR process as an advanced biological treatment process, an anoxic tank, an anaerobic tank, an aerobic tank, and an aerobic settling tank are basically used to remove nitrogen and phosphorous from sewage/wastewater.
In a typical biological sewage treatment process, the quality of treated water greatly depends on solid-liquid separation efficiency in a settling reservoir. That is, microorganisms take in organic matters and nutrient salts from sewage/wastewater within an aerobic biological reactor, and are grown, and the grown microorganisms are deposited in the form of sludge in a gravity settling reservoir, and are separated and removed from water. By the way, when settling efficiency is decreased according to operating conditions in a treatment process, the quality of discharged water may be degraded.
Meanwhile, a separation membrane technology for treating sewage/wastewater is constantly applied and expanded for the last twenty years, and is highly regarded as a reliable technology in sewage/wastewater recycling and advanced treatment fields. To address the above-described fundamental issues, a membrane bio reactor (MBR) process is introduced, which includes a filtering process using a separation membrane to replace a typical gravitational deposition process, and has advantages of a biological process and a separation membrane technology to compensate for disadvantages of a typical activated sludge process. Particularly, a submerged MBR process makes it possible to completely separate a solid and a liquid from each other and to obtain stable treated water, and thus, is steadily applied to a sewage treatment field, and performances thereof are improved.
Such a typical MBR process is disclosed in U.S. Pat. No. 5,192,456 that pertains to an activated sludge treating apparatus for treating wastewater, in which a plurality of KUBOTA filter membrane modules are vertically arranged in parallel at predetermined intervals within a treating tank, and treating target water stored in the treating tank is separated into solid and liquid by the filter membrane modules. In this US patent, secondary clarification by gravity settling is replaced with membrane separation. Other typical MBR processes and systems use bundled hollow fiber membranes such as GE (Zenon) or Econity membranes and modules. In an MBR plant for biologically removing nitrogen, a separate anoxic process may be performed, and then, an aeration process as an MBR process may be performed. In this case, mixed liquor is recycled from the aeration process to the anoxic process, so that nitrate is provided in the anoxic process with a certain number of bacteria maintained. Accordingly, in the anoxic process, nitrogen gas is supplied with oxygen from the nitrate provided through the circulation with the microorganisms, and is discharged. Nitrification occurs in the aeration process (MBR process).
FIG. 1 is a schematic view showing a typical total MBR system according to the related art.
Referring to FIG. 1, wastewater is introduced into an anoxic mixed tank, successively flows into an aerobic tank, and is treated in a dedicated aerated membrane filtration tank. Nitrified sludge is recycled from the membrane filtration tank through a recycling line back to the mixed cell. An MBR aerated tank including a plurality of porous thin membranes or hollow fiber membranes are disposed within the aerated membrane tank. Treated water purified through solid-liquid separation in the MBR is discharged, and waste sludge is discarded.
Since the typical MBR process today maintains a high mixed liquor suspended solid (MLSS) concentration up to 10,000 mg/L in aeration tanks and up to 18,000 mg/L in membrane tanks (WEF Manual of Practice No. 36), organic matter can be suitably treated in smaller bioreactors than required for suspended growth conventional activated sludge treatment systems. In addition, if a separation membrane corresponding to micro porous size is used, turbidity of treated water can be improved, and particulates such as colon bacilli can be efficiently removed, thereby improving the quality of treated water. Furthermore, since solids retention time (SRT) is typically high in MBRs (15 days or more), auto-oxidation of sludge may be achieved to somewhat reduce the production of waste sludge.
However, if an excessively high MLSS concentration is maintained in a MBR process reactor, the total phosphorous (TP) value of the treated effluent may rapidly increase. In addition, the MBR process includes a recycle operation from an aerobic tank to an anaerobic tank and an anoxic tank to remove nitrogen and phosphorous. In this case, if a large amount of dissolved oxygen and/or nitrate nitrogen is recycled, phosphorous removing efficiency can decrease. And if excessive dissolved oxygen is recycled to the anoxic tank, nitrogen removing efficiency can also decrease. Finally if high MLSS concentrations are maintained in a MBR process reactor, the aeration energy requirements can increase by 50 to 100% more than for conventional activated sludge processes. Considering that about 75% of MBR wastewater plant energy use is from aeration blowers (See WEF Manual of Practice No. 36), the major alpha factor reductions at high MLSS concentrations may cause MBR processes to have unsustainably high energy consumption. As a consequence it is essential when selecting suitable processes for staged upgrading planning in advance to MBR processes, that minimal increased MLSS concentrations should be planned and proven in advance before selecting suspended growth biological nutrient removal (BNR) processes that may require MBR upgrades to increase flow capacity and/or treated effluent quality in the future.
Meanwhile, U.S. Pat. No. 7,172,699 discloses an EIMCO wastewater treatment system for decreasing the volume of an aerobic tank. According to this patent, wastewater may be treated through nitrification/denitrification, aerobic, anoxic/simultaneous nitrification/denitrification, and oxic stages, and the volume ratio among the four stages is about 65:10:15:10 such that the volume of the aerobic tank is only 20% of the total wastewater system volume, thereby reducing aeration requirements.
To this end, a large site and a construction area are required. Moreover, additional process facilities for satisfying tighten effluent quality criteria are required or pre-existed construction systems should be demolished to convert to new MBR treatment facilities. Accordingly, a large site for the additional process facilities, and costs for building the new facilities are required making conversion to MBR Treatment capability not cost effective. It can be seen that conversion to MBR Treatment capability from suspended growth conventional activated sludge treatment systems can often be not cost effective. And even if it may be cost effective, conversion to MBR Treatment capability may be complex and require shutdown of the existing treatment plant and loss of treatment capacity for long periods while the conventional activated sludge treatment systems are converted to MBR Treatment systems.